1. Field of the Invention
The present invention relates to a flat display apparatus including a screen constituted of a group of light-emitting elements, such as organic electroluminescent (EL) elements. More specifically, the present invention relates to a technology for extending the light emission life of the light-emitting elements by suppressing degradation that occurs over time by improving the circuitry of the light-emitting elements.
2. Description of the Related Art
A popular flat panel display apparatus under development, such as an organic electroluminescent (EL) display, is constituted of pixels including light-emitting elements. An organic EL display is capable of displaying high-quality images having a wide viewing angle, a high-speed response, a wide range of color reproducibility, and high contrast. An organic EL display has a thin panel. These features of an organic EL display fulfill the demands placed on next-generation flat panel displays following liquid crystal displays and plasma displays.
It is known that light-emitting elements included in the pixels of an organic EL display degrade in accordance with the cumulative amount of light the light-emitting elements emit. In other words, the luminance of the light-emitting elements decreases over time. Extending the lifetime of a light-emitting element is a great challenge to be faced in developing organic EL displays.
At present, to use an organic EL display, for example, as a monitor for a television set, the light emission life of the light-emitting elements must be extended. However, the development of organic EL materials used for organic EL light-emitting elements require enormous time and cost. For this reason, the light emission life of light-emitting elements has not been extended dramatically. To develop an organic EL display that can be put to practical use in the near future, it is necessary to extend the light emission life of light-emitting elements to a practical level by providing an improved driving method of the light-emitting elements in addition to extending the light emission life of the light-emitting elements by developing new materials.
Technologies for extending the life of an organic EL display by improving the circuitry are disclosed, for example, in the following documents:
Japanese Unexamined Patent Application Publication Nos. 07-036410, 2003-150110, 2002-169509, 08-248934, 2000-356981, 2003-195816, 2003-122305, and 2003-255895.
Similar to a cathode-ray tune (CRT) display, the user is capable of controlling the luminance of the screen and the contrast of an organic EL display. More specifically, the luminance of the screen can be controlled by changing the duty ratio. The duty ratio is a value specifying the proportion of the light-emitting time of a light-emitting element in one frame period. The life of a light-emitting element is extended by applying duty control in Japanese Unexamined Patent Application Publication Nos. 2003-195816 and 2003-122305. By controlling the duty ratio, the life of the light-emitting element can be extended by shortening the light-emitting time of the light-emitting element when an image of a frame is bright and the life of the light-emitting element can be extended by extending the light-emitting time of the light-emitting element when an image of a frame is dark. According to such known methods, only the duty ratio is controlled, and the life of a light-emitting material can only be extended by controlling the light-emitting time of the light-emitting element by only controlling the duty ratio. Therefore, the life of the light-emitting element has not been extended to a practical level.
According to Japanese Unexamined Patent Application Publication No. 07-036410, the amount of change in a driving voltage of a light-emitting element is detected and a constant current signal is controlled in accordance with the amount of change. According to Japanese Unexamined Patent Application Publication No. 2003-150110, a reverse bias is applied while an EL element is not illuminating so as to prevent degradation of the EL element. According to Japanese Unexamined Patent Application Publication No. 2003-255895, a reverse bias is applied to an EL element in synchronization with the driving of the EL element such as writing-in, emitting, and deleting an image signal so as to extend the life of the EL element. According to Japanese Unexamined Patent Application Publication No. 2002-169509, degradation of a light-emitting element is prevented by reducing the amount of unnecessary light-emitting time by using a pixel circuit that is capable of actively discharging the retention volume of the pixel. According to Japanese Unexamined Patent Application Publication No. 08-248934, burn-in, which is a type of degradation, is prevented by slightly displacing the display position of a screen for each frame so that one area is illuminated for a long period of time. According to Japanese Unexamined Patent Application Publication No. 2000-356981, the speed of degradation of a light-emitting element is reduced by decreasing the luminance of the light-emitting element based on degradation calculations based on a measurement of the amount of time an image is displayed on a display unit.